I think another possibility is that you are getting the output of a screen from the manufacturer. Making the capacitors then pulling those that are 1%, 5%, and 10% from the mix. This leaves the ends of the screen available to those purchasing the 20% parts. It has been years, but I did experience this from a maker of film caps. Sometimes we would get all parts above the nominal other times all below the nominal. Never, nominal. When we paid for 5%, we got parts around nominal. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea Sent: Tuesday, July 01, 2008 11:18 AM To: [log in to unmask] Subject: Re: [TN] Ceramic Cap Value Hi Ken, We've seen this phenomenon twice in the past 4 months, high rate of ICT failure on ceramic caps and had the same issue, replacing the parts sometimes worked and sometimes didn't. We even have a painful open case on this matter. I need more clarifications from the group regarding this issue and how to get around it. The paper on aging asks to take the phenomenon into account when designing the circuitry. Does this mean the high failure rate Ken and I witnessed is borderline design? Can it be marginal testing method also? Can it be anything else that design or testing? Thanks, Ioan -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Bruce Tostevin Sent: Monday, June 30, 2008 9:37 AM To: [log in to unmask] Subject: Re: [TN] Ceramic Cap Value Ken, As John Maxwell pointed out, it sounds like you may be seeing the normal effects of capacitor dielectric aging. It's a common stumbling block with parts at incoming inspection, test, etc. There's a pretty good one page summary with a handy formula at: http://www.atceramics.com/technicalnotes/circuit_designer.asp Click on Dielectric Aging Phenonema. The aging aspects of ceramic capacitors are also covered in EIA-521. A quick way to see if this is what's going on with your parts is to preheat then a little bit and float a few parts in a solderpot for 5-10 seconds and remeasure them after they cool. That'll usually bring the value up enough to confirm the issue. Bruce Tostevin Benchmark Electronics Hudson, New Hampshire -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Ken Bloomquist Sent: Friday, June 27, 2008 5:14 PM To: [log in to unmask] Subject: [TN] Ceramic Cap Value Happy Friday, I need some help understanding how you test for the value of ceramic capacitors. We build a board that has some 0603, .470 uF ceramic caps, +/- 10%. The assembly that these parts are on has been having a higher failure rate at our ICT then we've seen in the past. We tracked the failure to this cap which had a low reading. We replaced the cap and the unit passed ICT just fine. We measured the cap using our HP 4262A LCR Meter and sure enough it was reading low, around .398 uF. We pulled some new parts off the reel and they read low also ranging anywhere from .398 to .425. We suspected that we had a bad reel of parts even thought they were from a respected cap manufacturer. Being curious we tested some more reels from different lot codes and found the same thing. Being even more curious we tested some other parts from the same manufacturer and found them to be low by around 5% - 7% and in some cases slightly below 10%. We then tested some other parts from a different manufacturer and they tested low also. Now we are suspecting our test equipment so we went to an independent lab and had them tested on an Andeen Hagerling 2500A Bridge. Guess what, we got the same low readings. After all that my question is, what are we doing wrong to get such low readings on these ceramic caps? It just doesn't seem likely that four different parts from three different major suppliers tested on two different pieces of equipment would all be that far down on the low end. Any education in capacitor testing and reading would surely be appreciated. Best regards, KennyB --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------